Idaho basin outlook report

26 Pages·1993·2.3 MB·English

by  United States. Soil Conservation Service

Historic, Archive Document
Do not assume content reflects current
scientific knowledge, policies, or practices.

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aTD22^ Idaho
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Soil
Conservation
Service Basin Outlook Report
A
January
1
Basin Outlook Reports
and
Federal - State - Private
Cooperative Snow Surveys
For more watersupply and resource management information, contact:
Your local Soil Conservation Service Office
or
Soil Conservation Service
Snow Surveys
3244 Elder Street, Room 124
Boise ID 83705-4711
[phone]
How forecasts are made
Most of the annual streamflow in the Western United States originates as snowfall that
has accumulated high in the mountains during winter and early spring. As the snowpack
accumulates, hydrologists estimate the runoff that will occur when it melts. Predictions are
based on careful measurements of snow water equivalent at selected index points.
Precipitation, temperature, soil moisture and antecedent streamflow data are combined with
snowpack data to prepare runoff forecasts. Streamflow forecasts are coordinated by Soil
Conservation Service and National Weather Service hydrologists. This report presents a
comprehensive picture of water supply conditions for areas dependent upon surface runoff.
It includes selected streamflow forecasts, summarized snowpack and precipitation data,
reservoir storage data, and narratives describing current conditions.
Snowpack data are obtained by using a combination of manual and automated SNOTEL
measurement methods. Manual readings of snow depth and water equivalent are taken at
locations called snow courses on a monthly or semi-monthly schedule during the winter. In
addition, snow water equivalent, precipitation and temperature are monitored on a dally basis
and transmitted via meteor burst telemetry to central data collection facilities. Both monthly
and daily data are used to project snowmelt runoff.
Forecast uncertainty originates from two sources: uncertainty of future hydrologic and
(1)
climatic conditions, and (2) error in the forecasting procedure. To express the uncertainty in
the most probable forecast, four additional forecasts are provided. The actual streamflow
can be expected to exceed the most probable forecast 50% of the time. Similarly, the actual
streamflow volume can be expected to exceed the 90% forecast volume 90% of the time. The
same is true for the 70%, 30%, and 10% forecasts. Generally, the 90% and 70% forecasts
reflect drier than normal hydrologic and climatic conditions; the 30% and 10% forecasts
reflect wetter than normal conditions. As the forecast season progresses, a greater portion
of the future hydrologic and climatic uncertainty will become known and the additional
forecasts will move closer to the most probable forecast.
All programs and services of the USDA Soil Conservation Service are offered on a nondiscriminatory basis,
without regard to race, color, national origin, religion, sex, age, marital status, or handicap.
IDAHO WATER SUPPLY OUTLOOK REPORT
JANUARY
1993
1,
SUMMARY
After almost six years of drought conditions, Idaho’s mountain snowpack is off to a
promising start for the 1993 water year. Snowpacks throughout the state are near or
above normal for January 1. Soil moisture and ground water conditions are very dry,
however, and could have an adverse effect on runoff. Reservoir storage will play a
major role in the water supply for 1993. Many reservoirs across southern Idaho are
nearly empty, and water users will be almost entirely dependent upon snowpack and
the resulting runoff for their water supply next summer. Over half of the winter
accumulation season still lies ahead, and the next three months will determine the fate
of Idaho’s 1993 water supply.
SNOWPACK
Snowfall started around the first of November in Idaho, and frequent storms increased
the mountain snowpacks during the early winter months. Powder snow and excellent
coverage have delighted skiers throughout the state. Lower elevation srowpack
throughout most of the state is well above average. In a reversal of previous years’
trends, southern Idaho snowpacks are among the best in the state. Snowpacks
currently range from 130 to 155% of average across the southern edge of the state,
with the exception of the Bear River area which reports near average conditions. The
northern two-thirds of the state reports 85 to 120% of average snowpacks. The upper
Snake basin in Wyoming reports 96% of average snowpack. By January 1,
approximately 40% of the season’s snowpack is on the ground. With over half of the
—
winter season still ahead, there is plenty of time for snowpack conditions to change
for better or worse!
PRECIPITATION
The water year began with very little mountain rainfall prior to the beginning of snow
accumulation. As a result, most soils are very dry under the mountain snowpack and
could absorb significant snowmelt, reducing runoff yield. Overall, October was dry
in the Panhandle and wet in the south, especially in the central mountains. This trend
reversed in November as mountain precipitation in southern Idaho turned to a drier
pattern. During December, precipitation was near or above normal throughout most
of the state with well above average precipitation in southwest and southcentral Idaho.
Water year to date mountain precipitation is around 90% of average in northern Idaho
and the upper Snake River basin in Wyoming, near average in the central mountains
and Bear ^ver basin, and 120% of average in the south. Temperatures so far this
winter have been below normal. The months of November, December, and January
A
combined account for nearly half of the average annual mountain precipitation. wet
January will almost certainly be needed to ensure adequate water supplies in the
summer of 1993.
RESERVOIRS
Reservoir carryover storage is one of the few known quantities influencing this
NOT M^y
summer’s water supply... and the news is good. reservoirs in soutHem
Idaho are holding record low storage amounts as a result of the sus^n^ drought of
the last several years. Northern Idaho reservoirs are in somewhat better sHaj^.
Reservoirs of specific concern, holding less than 25 percent of capacity; include t^^^
Boise system, Little Wood, Mackay, Oakley, S^mbn Falls, Owyhee, Be^ t^e; WA
Montpelier Creek reservoirs. As usual, storage in the upper Sn^e ^d Payette
system is somewhat better; these systems report 32 and 47% of capacity, r^pect^ivel^
Some of the storage in the Payette system will be used to back fill Browhl^
Reservoir, so these figures should drop as water is released.
_ ^ ^ _
Note: SCS reports reservoir information in terms of ‘usable’ contents, which incliiides
both active and inactive storage. Other operators may report reservoir contents ih
different terms.
STREAMFLOW
October through December streamflow was below to well below normal tbrSughout
Idaho, a continuing sign of low soil moisture conditions due to the extended drought.
Fall streamflow was 50 to 70 percent of average across central and southern Idaho arid
70 to 80 percent of average in northern Idaho and the upper Snake. As a result,
reservoirs are refilling slower than normal and many operators have r^iiced the
outflows to bare minimums needed for protection of fisheries., Sfieamflpw for©^&
for the coming spring and summer are in the near average range throughout Id^o
with the exception of the Owyhee, Bruneau, Oakley and Salmon Falls basins iri
southern and southwestern Idaho where abundant snowpack has produced forecasts
well above normal for the first time in many years.
Panhandle Region
January 1993
1,
Mountain Snowpack (inches) Mountain Precipitation
PANHANDLE REGION PANHANDLE REGION
*-
CURRENT MONTHLY
->l<
AVERAGE WATER YEAR TO DATE
250.
MAXIMUM
MINIMUM
200.
Ol50_
f
A
V
el00.
r
a
g
50.
OCT NOV DEC
Based on selected SNOTEL sites
WATER SUPPLY OUTLOOK
The Panhandle region received slightly below normal precipitation during October
and November and near normal precipitation during December. Mountain
precipitation for the water year stands at 88% of average. Several major storm
systems moved through the Panhandle region during December and produced blizzard
conditions in the lower elevation areas. Snowpacks are well above normal in the
lower elevations, and near average in the higher elevations. Currently, snowpacks
range from 138% of average in the lower elevation Palouse basin to 97% of average
on the Pend Oreille basin. Streamflow forecasts range from 84% of average for the
inflow to Pend Oreille Lake to 108% of average for Coeur d’Alene River.
r
PANHANDLE REGION
Streamflow Forecasts January 1, 1993
«====== Drier ======= Future Conditions ======== Wetter
.....
Forecast Point
Period 90% 70% 50% (Most Probable) 30% 10% 30-Yr Avg.
(1000AF) (1000AF) (1000AF) (% AVG.) (1000AF) (1000AF) (1000AF)
KOOTENAI at Leonia (1,2) APR-JUN 3120 4430 5020 88 5610 6920 5701
APR-JUL 3970 5600 6340 88 7080 8710 7199
APR-SEP 4550 6420 7270 88 8120 9990 8275
CLARK FK at Whitehorse Rpds (1,2) APR-JUN 4660 7560 8870 88 10200 13100 10050
APR-JUL 5470 8860 10400 89 11900 15300 11730
APR-SEP 5970 9710 11400 88 13100 16800 12910
PEND OREILLE LAKE inflow (1,2) APR-JUN 4830 8320 9910 87 11500 15000 11390
APR-JUL 6020 9720 11400 87 13100 16800 13150
APR-SEP 6610 10700 12500 87 14300 18400 14370
PRIEST nr Priest River (1,2) APR-JUL 390 605 700 86 795 1010 814
APR-SEP 420 645 750 86 855 1080 868
COEUR D'ALENE at Enaville (1) APR-JUL 395 665 785 102 905 1180 769
APR-SEP 415 695 825 102 955 1240 809
ST. JOE at Calder APR-JUL 780 970 1100 94 1230 1420 1169
APR-SEP 730 1030 1160 94 1290 1580 1237
SPOKANE nr Post Falls (1,2) APR-JUL 785 2010 2570 98 3130 4360 2627
APR-SEP 950 2090 2670 98 3250 4410 2720
PANHANDLE REGION PANHANDLE REGION
Reservoir Storage (1000 AF) - End of December Watershed Snowpack Analysis - January 1, 1993
Usable *** Usable Storage *** Nunber This Year as % of
Reservoi Capacity This Last Watershed of =====
Year Year Avg Data Sites Last Yr Average
HUNGRY HORSE 3451.0 1438.0 2110.0 2586.0 Kootenai ab Bonners Ferry 24 80 73
FLATHEAD LAKE 1791.0 1021.0 1022.0 1305.0 Moyie River 2 78 61
NOXON RAPIDS 335.0 311.2 322.3 317.1 Clark Fork River 44 81 94
PEND OREILLE 1561.3 486.4 541.2 744.9 Priest River 4 111 97
COEUR D'ALENE 238.5 44.5 140.6 130.5 Pend Orei1le River 60 88 97
PRIEST LAKE 119.3 51.8 21.8 54.8 Rathdrum Creek 5 299 129
Hayden Lake 0 0 0
Coeur d'Alene Hiver 5 99 104
St. Joe River 2 97 107
Spokane River 12 129 112
Palouse River 1 200 138
* 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual flow will exceed the volumes in the table.
The average is computed for the 1961-1990 base period,
(1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels.
(2) - The value is natural flow - actual flow may be affected by upstream water management.
Clearwater River Basin
January 1993
1,
Mountain Snowpack (inches) Mountain Precipitation
CLEARWATER RIVER BASIN CLEARWATER RIVER BASIN
>|<— ->|< CURRENT
—
- AVERAGE WATER YEAR TO DATE
250
MAXIMUM
n
t
0 150
f
A
V
100
e
r
a
g
® 50
0
OCT NOV DEC
Based on selected SNOTEL sites
WATER SUPPLY OUTLCX)K
The water year started with below normal mountain precipitation in October and then
improved with near average precipitation falling in November and December.
Mountain precipitation for the water year is still slightly below average at 93% of
average. Overall, the Clearwater basin is reporting a near normal snowpack at 106%
of average. The snowpack in the lower elevations is above normal while the
snowpack in the higher elevations is near normal. Dworshak Reservoir storage is
75% of capacity which is normal for this time of year. Streamflow forecasts range
from 90 to 110% of average for the Clearwater basin. Current conditions indicate
that water supplies should be adequate in the Clearwater basin during 1993.
r
CLEARWATER RIVER BASIN
Streamflow Forecasts - January 1, 1993
«====== Drier ====:== Future Conditions ======= Wetter
Forecast Point Forecast
|
Period 90% 70% 50% (Most Probable) 30% 10% 30-Yr Avg.
| 1 | 1
(1000AF) (1000AF) (1000AF) (% AVG.) (1000AF) (100QAF) (1000AF)
1 1 1
DWORSHAK RESERVOIR inflow (1) APR-JUL 1720 2580 2970 110 3360 4220 2700
1 1
APR-SEP 1830 2740 3160 110 3580 4490 2875
1 1
1 1
1 1
CLEARWATER at Orofino (1) APR-JUL 2050 3570 4260 90 4950 6470 4718
1 1
APR-SEP 2170 3770 4500 90 5230 6830 4976
1 I
1 1
1 1
CLEARWATER at Spalding (1,2) APR-JUL 3900 6470 7640 100 8810 11400 7618
1 1
APR-SEP 4110 6830 8070 100 9310 12000 0052
1 1
1 1
CLEARWATER RIVER BASIN CLEARWATER RIVER BASIN
Reservoir Storage (1000 AF) - End of December Watershed Snowpack Analysis - January 1, 1993
Usable *** Usable Storage *** Number This Year as % of
|
Reservoi Capacity This Last Watershed
I
Year Year Avg Data Sites Last Yr Average
I
DWORSHAK 3467.8 2627.0 1453.1 2431.0 North Fork Clearwater 11 94 105
|
1
1
Lochsa River 4 89 98
1
1
1
Selway River 5 94 108
1
1
1
Clearwater Basin Total 19 94 106
1
90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual flow will exceed the volumes in the table.
The average is computed for the 1961-1990 base period.
(1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels.
(2) - The value is natural flow - actual flow may be affected by upstream water management.